1. Field of the Invention
The present invention relates to an automatic focus adjusting device which detects a focus condition of an objective lens of a camera by receiving a light form an object passed through an objective lens and drives the objective lens based upon the focus condition detected.
2. Description of the Prior Art
There has been known a focus condition detecting device in which two object images are formed by reimaging two object light bundles having been passed through a first and second areas of an objective lens being symmetric to each other with respect to the optical axis 18 thereof, a positional relationship between these two object images is calculated based upon correlation calculation thereabout, and then, focus condition data such as a defocus amount of the detected focus position from a predetermined focus position i.e., an in-focus position, and the direction of the defocus are obtained.
A typical optical system for the focus condition detecting device of this type is shown in FIG. 1.
As shown in FIG. 1, this optical system includes a condenser lens 6 being arranged on the predetermined focal plane 4 set behind the objective lens 2 equivalent to a film sequence of a camera or on a plane behind the predetermined focal plane 4 and two reimaging lenses 8 and 10 being arranged rearside of the condenser lens 6 and, there are arranged two image sensors 12 and 14 on respective focal planes of the reimaging lenses 8 and 10. Each of these image sensors is constituted by a charge integration or light intergration type photo detecting device such as a CCD (charge coupled device).
As shown in FIG. 2 schematically, two object images reimaged on the image sensors are approached to each other with respect to the optical axis 18 of the objective lens 2, if an object image is formed front side of the predetermined focal plane 4, and are apart from each other with respect to the optical axis 18 if the object image is formed rearside thereof. If the object image is formed just on the predetermined focal plane 4, a distance between two corresponding points of the reimaged two object images becomes equal to a specific value which is determined by the composition of the optical system. Accordingly, the focus condition can be obtained, in principle, by detecting a distance between two corresponding points of the reimaged two object images.
In an automatic focus adjusting device of a camera installing such an optical system for detecting a focus condition as mentioned above, a control circuit constituted by a micro-computer is provided for controlling a series of sequence of integration of light amount of an object image by each CCD image sensor, detection and calculation of a focus condition with use of outputs of the image sensor (calculation of defocus amount), driving operation of the objective lens based upon the defocus amount detected and stopping the objective lens at an in-focus position thereof (if a shutter release button is pushed down for a shutter release).
The automatic focus adjusting device repeats the abovementioned automatic focus adjusting sequence, even when the objective lens is moved close to an in-focus position thereof, in order to set the objective lens at an in-focus position accurately.
In JP-A Nos. 78823/1981 or 156028/1985, there is disclosed a focus condition detecting system wherein focus condition detecting operations are carried out even when the objective lens is moving in order to enhance the accuracy in the focus condition detection.
Meanwhile, in the automatic focus adjusting device, after a defocus amount is obtained by one focus condition detecting operation in the case that an object is approaching to the camera or aparting therefrom, and if the objective lens is moved to the in-focus position determined according to the defocus amount obtained, the focus condition of the objective lens adjusted has become inaccurate, since the object has been moved during the focus adjustment.
Accordingly, it is desirable to shorten calculation time needed for the calculation of defocus amount as short as possible in order to obtain an exact in-focus condition even in the case that an object is moving.
Conventionally, an in-focus position is determined in such a manner that a lens driving amount being measured from the center point of an integration to finishment of a focus condition detecting operation is compared with a defocus amount detected and the objective lens is stopped at the timing that the former becomes equal to the latter.
However, in this method, since the center point of an integration has to be determined as a point at which an amount of integrated charge becomes equal to one-half of a predetermined value, the constitution of the focus condition detecting circuit becomes complex and this invites a higher cost.